To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Current starting price: 50 euros.
2. Coin condition: 70 (MBC-), indicating an acceptable but not excellent condition.
3. Auction history:
- It has been auctioned 4 times in the past, of which it has sold 3 times.
- The average starting price in previous auctions was 91.32 euros, and the average hammer price was 163.89 euros.
- Previous hammer prices were 160 euros (2012), 262.73 euros (2013), and 68.93 euros (2015).

4. Price trends:
- The highest hammer price was 262.73 euros, but this corresponds to a condition of 90, which is significantly better than the current condition of 70.
- The lowest hammer price was 68.93 euros, which corresponds to a condition of 75.

Given that the current condition of the coin is 70, which is lower than the conditions of previous auctions, it is reasonable to adjust the expected hammer price downward. However, the starting price of 50 euros is quite low compared to previous hammer prices.

Estimated maximum purchase price:
- Considering the average hammer price of 163.89 euros and adjusting for condition, a reasonable maximum purchase price could be in the range of 80 to 120 euros. This is based on the idea that, although the condition is not the best, the interest in the coin and its auction history suggest it could reach a higher price than the starting price.

Therefore, an estimated maximum purchase price could be 100 euros.